Crosslinkable rubber composition

ABSTRACT

A rubber composition comprising a combination of a nitrile rubber and an epilhalohydrin rubber is cross-linked to give a crosslinked product, by using at least two kinds of crosslinking agents selected from a sulfur-containing crosslinking agent, a mercaptotriazine crosslinking agent and a 2,3-dimercaptoquinoxaline crosslinking agent.

TECHNICAL FIELD

This invention relates to a crosslinkable rubber composition giving acrosslinked product having good ozone resistance, and comprising, asrubber ingredient, a nitrile rubber and an epihalohydrin rubber, and, ascrosslinking agent, at least two kinds of crosslinking agents selectedfrom a sulfur-containing crosslinking agent, a mercaptotriazinecrosslinking agent and a 2,3-dimercaptoquinoxaline crosslinking agent.

BACKGROUND ART

A blend (usually referred to as “polyblend”) of anacrylonitrile-butadiene copolymer rubber (NBR) with a vinyl chlorideresin (PVC) has heretofore been used as a material for producing acrosslinked rubber product for which an oil resistance and an ozoneresistance are required. However, when PVC is incinerated, dioxin whichis a pathocrinia substance is produced unless the incinerationtemperature is carefully and precisely controlled. Thereforeincineration of PVC is avoided and the demand for PVC is diminishing.

As a substitute material for the polyblend which has good oil resistanceand ozone resistance, a blend of NER with an epthalohydrin rubber isknown. NBR and an epihalohydrin rubber exhibit different crosslinkingmechanisms and thus the rubber blend is impossible to crosslink with asingle crosslinking agent. Attempts for co-crosslinking the two rubbersby using a combination of crosslinking agents which are effective forcrosslinking the respective rubbers have been made.

For example, a method of co-crosslinking a blend of a diene elastomerand an epihalohydrin polymer was proposed wherein an organicpolysulfide, which is a sulfur-donating compound capable of crosslinkinga diene rubber, and 2-mercaptoimidazoline or thiourea, which is acrosslinking agent for an epihalohydrin polymer, and an oxide ofmagnesium, calcium, zinc or lead are incorporated together in the blend(Japanese Examined Patent Publication No. S50-4032). However, where anoxide of magnesium, calcium or zinc is incorporated, the resultingco-crosslinked product has poor thermal aging resistance and dryphysical properties, especially tensile strength, and the rate ofcrosslinking for the production thereof is low and the productivity ispoor. Lead oxide is toxic and its use is restricted. Therefore theabove-proposed method of co-crosslinking diene elastomer/epihalohydrinpolymer has poor practical use.

An attempt of co-crosslinking a blend of NBR and epihalohydrin rubberwas made wherein sulfur as a crosslinking agent and tetramethylthiurammonosulfide or benzothiazolyl disulfide as a crosslinking acceleratorare used in combination. But, the crosslinked product has poor ozoneresistance.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a crosslikable rubbercomposition which is capable of giving a crosslinked product exhibitingimproved oil resistance and ozone resistance, as a substitute materialfor the conventional polyblend.

The present inventors made extensive researches to achieve theabove-mentioned object and found that a blend of nitrile rubber and anepihalohydrin rubber can be co-crosslinked by using at least two kindsof crosslinking agents selected from a sulfur-containing crosslinkingagent, a mercaptotriazine crosslinking agent and a2,3-dimercaptoquinoxaline crosslinking agent, to give a crosslinkedproduct exhibiting improved oil resistance and ozone resistance.

Thus, in accordance with the present invention, there is provided acrosslinkable, rubber composition comprising, as rubber ingredient, anitrile rubber and an epihalohydrin rubber, and, as crosslinking agent,at least two kinds of crosslinking agents selected from asulfur-containing crosslinking agent, a mercaptotriazine crosslinkingagent and a 2,3-dimercaptoquinoxaline crosslinking agent; and furtherthere is provided a crosslinked product.

BEST MODE FOR CARRYING OUT THE INVENTION

[Crosslinkable Rubber Composition]

The crosslinkable rubber composition of the present invention comprises,as rubber ingredient, a nitrile rubber and an epihalohydrin rubber, and,as crosslinking agent, at least two kinds of crosslinking agentsselected from a sulfur-containing crosslinking agent, a mercaptotriazinecrosslinking agent and a 2,3-dimercaptoquinoxaline crosslinking agent.

Rubber Ingredient

The rubber ingredient used in the present invention is a combination ofa nitrile rubber and an epihalohydrin rubber.

(i) Nitrile Rubber

Nitrile rubber contained in the rubber ingredient used in the presentinvention is prepared by copolymerizing an unsaturated nitrile compoundsuch as acrylonitrile and methacrylonitrile, with at least oneconjugated diene monomer such as 2-methyl-1,3-butadiene, 1,3-butadiene,1,3-pentadiene and 2-chloro-1,3-butadiene. If desired, at least onemonomer copolymerizable with the unsaturated nitrile compound andconjugated diene monomer may be further copolymerized. Suchcopolymerizable monomers include, for example, unsaturated carboxylicacids such as acrylic acid, methacrylic acid, maleic acid, fumaric acidand itaconic acid; acrylic acid esters and methacrylic acid esters, suchas methyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate,2-ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate,glycidyl acrylate, glycidyl methacrylate, 2-hydroxyethyl acrylate and2-hydroxypropyl methacrylate; and acrylic acid amides, methacrylic acidamides and their derivatives, such as acrylamide, methacrylamide,N-methylolacrylamide, N-methylolmethacrylamide, N-butoxymethylacrylamideand N-butoxymethylmethacrylamide. The content of the copolymerizablemonomer units in the copolymer is preferably in the range of 0.1 to 15%by weight. A preferable nitrile rubber is an acrylonitrile-butadienecopolymer rubber (NBR).

The amount of the unsaturated nitrile compound (usually referred to as“amount of bound unsaturated nitrile”) in the nitrile rubber is notparticularly limited, but is preferably in the range of 10 to 55% byweight. The amount is appropriately chosen so that the performancesrequired for the crosslinked product, such as oil resistance and coldresistance, are satisfied.

The nitrile rubber preferably has a Mooney viscosity (ML₁₊₄, 100° C.) inthe range of 25 to 140, more preferably 45 to 90.

(ii) Epihalohydrin Rubber

Epihalohydrin rubber contained in the rubber ingredient used in thepresent invention includes a homopolymer of epihalohydrin such asepichlorohydrin or epibromohydrin, a copolymer of different kinds ofepihalohydrin, and a copolymer of epihalohydrin and a copolymerizablemonomer. The copolymerizable monomer includes, for example, alkyleneoxides such as ethylene oxide, propylene oxide and butylene oxide; andunsaturated epoxides such as allyl glycidyl ether, glycidyl acrylate,glycidyl methacrylate, butadiene monoxide and vinylhexene monoxide.These copolymerizable monomers may be used as a combination of at leasttwo thereof. As preferable examples of the copolymer of epihalohydrinwith a copolymerizable monomer or monomers, there can be mentionedepihalohydrin (100–50% by mole)-ethylene oxide (0–50% by mole)copolymers, epihalohydrin (10–60% by mole)-ethylene oxide (0–50% bymole)-propylene oxide (0–30% by mole) terpolymers, epihalohydrin (90–70%by mole)-allyl glycidyl ether (10–30% by mole) copolymers, andepichlorohydrin (40–80% by mole)-ethylene oxide (50–20% by mole)-allylglycidyl ether (2–20% by mole) terpolymers.

The epihalohydrin rubber preferably has a Mooney viscosity (ML₁₊₄, 100°C.) in the range of 30 to 140, more preferably 50 to 80.

(iii) Other Rubbers

If desired, the rubber ingredient used in the present invention maycontain rubbers other than the above-mentioned nitrile rubber andepihalohydrin rubber, provided that the characteristics of thecrosslinkable rubber composition and the crosslinked product are notbanefully influenced. Such optional rubbers include, example, naturalrubber, polybutadiene rubber, polyisoprene rubber, styrene-butadienerubber, chloroprene rubber and acrylic rubber.

(iv) Proportion of Rubbers

Optimum proportion of the nitrile rubber and the epihalohydrin rubber isdetermined depending upon the properties desired for the crosslinkedproduct of the present invention, such as oil resistance ozoneresistance. The amount of the nitrile rubber is preferably in the rangeof 20 to 80% by weight, more preferably 50 to 70% by weight, based onthe total weight of the rubber ingredient. The amount of theepihalohydrin rubber is preferably in the range of 80 to 20% by weight,more preferably 50 to 30% by weight, based on the total weight of therubber ingredient. The amount of optional rubbers is such that thecharacteristics of the crosslinkable rubber composition of the presentinvention and the crosslinked product of the present invention are notdeteriorated, and is preferably not larger than 50% by weight, morepreferably not larger than 30% by weight and especially preferably notlarger than 10% by weight, based on the total weight of the rubberingredient.

Crosslinking Agent

The crosslinking agent used in the present invention is a combination ofat least two kinds of crosslinking agents selected from asulfur-containing crosslinking agent, a mercaptotriazine crosslinkingagent and a 2,3-dimercaptoquinoxaline crosslinking agent. By using atleast two kinds of the specified crosslinking agents, the rubberingredient used can be co-crosslinked to give a crosslinked producthaving excellent ozone resistance, as compared with a conventionalproduct crosslinked by using each crosslinking agent alone.

(i) sulfur-Containing Crosslinking Agent

The sulfur-containing crosslinking agent used in the present inventionhas a function of crosslinking a nitrile rubber. The sulfur-containingcrosslinking agent includes sulfur and sulfur-donating compounds. Asspecific examples of the sulfur-donating compounds, there can bementioned thiuram compounds such as tetramethylthiuram disulfide,tatraethylthiuram disulfide and dipentamethylenethiuram tetrasulfide,and morpholine disulfide.

A crosslinking accelerator is preferably used in combination with thesulfur-containing crosslinking agent. Conventional crosslinkingaccelerators used in combination with sulfur-containing crosslinkingagents for crosslinking diene rubbers can be used for crosslinking therubber ingredient used in the present invention. As preferable examplesof the crosslinking accelerator, there can be mentioned thiuramaccelerators such as tetramethylthiuram monosulfide, tetramethylthiuramdisulfide, tetraethylthiuram monosulfide and tetraethylthiuramdisulfide; and thiazole accelerators such as benzothiazolyl disulfide,N-cyclohexyl-2-benzothiazole-sulfenamide andN-oxydiethylene-2-benzothiazolesulfenamide. These crosslinkingaccelerators may be used either alone or as a combination of at leasttwo thereof.

An accelerator activator can be used in combination with thecrosslinking accelerator. The accelerator activator includes, forexample, fatty acids such as stearic acid, oleic acid and lauric acidand their metal salts; metal oxides such as zinc oxide and magnesiumoxide; metal hydroxides such as calcium hydroxide; and zinc carbonate.Of these, a combination of magnesium oxide and stearic acid ispreferable because magnesium oxide functions as an acid acceptor as wellas an accelerator activator for an epihalohydrin rubber when thecrosslinkable rubber composition is crosslinked.

The amount of the sulfur-containing crosslinking agent is preferably inthe range of 0.1 to 3 phr, more preferably 0.2 to 3 phr, and especiallypreferably 0.3 to 1.5 phr, as expressed as amount of sulfur. By the term“phr” used herein, we mean “parts by weight per hundred parts by weightof the rubber ingredient”.

The amounts of the crosslinking accelerator and the acceleratoractivator are suitably determined depending upon the particular kindsthereof so that storage stability and crosslinking rate of acrosslinkable rubber composition and the properties required for acrosslinked product are attained. The amount of the crosslinkingaccelerator is preferably in the range of 0.5 to 5 phr. The amount ofthe accelerator activator is preferably in the range of 0.1 to 3 phrwhere it is stearic acid, and preferably in the range of 0.5 to 10 phrwhere it is magnesium oxide, which is relatively large because magnesiumoxide functions as an acid acceptor in addition to an acceleratoractivator.

(ii) Mercaptotriazine Crosslinking Agent

The mercaptotriazine crosslinking agent used in the present inventionincludes dimercaptotriazine compounds and trimercaptotriazine compounds,both of which have a function of crosslinking an epihalohydrine rubber.As specific examples of the mercaptotriaizine crosslinking agent, therecan be mentioned 2,4,6-trimercapto-s-triazine,2-methyl-4,6-dimercapto-s-triazine,2-methylamino-4,6-dimercapto-s-triazine and2-diethylamino-4,6-dimercapto-s-triazine. Of these,2,4,6-trimercapto-s-triazine is preferable because it is easilyavailable.

The amount of the mercaptotriazine crosslinking agent is preferably inthe range of 0.1 to 10 phr, more preferably 0.2 to 8 phr and especiallypreferably 0.5 to 3 phr.

An acid acceptor is preferably used in combination with themercaototriazine crosslinking agent. In view of control of crosslinkingrate and thermal stability of a crosslinked product, the acid acceptorpreferably includes oxides, hydroxides, carbonates, carboxylic acidsalts, silicyc acid salts, boric acid salts and phosphorous acid saltsof metals of group II of the periodic table; oxides, basic carbonic acidsalts, basic carboxylic acid salts, basic phosphorous acid salts, basicsulfurous acid salts and tribasic sulfuric acid salts of metals of groupIVA of the periodic table; and hydrotalcites.

As specific examples of the acid acceptor, there can be mentionedmagnesium oxide, magnesium hydroxide, barium hydroxide, magnesiumcarbonate, barium carbonate, quicklime, calcium hydroxide, calciumcarbonate, calcium silicate, calcium stearate, zinc stearate, calciumphthalate, calcium phosphite, zinc oxide, tin oxide, litharge, dibasiclead phthalate, dibasic lead carbonate, tin stearate, basic leadphosphite, basic tin phosphite, basic lead sulfite and tribasic leadsulfate. Of these, magnesium oxide is preferable in view of thecrosslinking performance and properties of a crosslinked product.

Hydrotalcites are compounds represented by the formula:Mg_(x)Al_(y)(OH)_(2x+3y−2)CO₃.wH₂O wherein x is an integer of 1 to 10, yis an integer of 1 to 5 and w is a positive number. As specific examplesof the hydrotalcites, there can be mentionedMg_(4.5)Al₂(OH)₁₃CO₃.3.5H₂O. Mg_(4.5)Al₂(OH)₁₃CO₃,Mg₄Al₂(OH)₁₂CO₃.3.5H₂O, Mg₆Al₂(OH)₁₆CO₃.4H₂O. Mg₅Al₂(OH)₁₄CO₃.4H₂O andMg₃Al₂(OH)₁₀CO₃.1.7H₂O.

The amount of the acid acceptor is preferbly in the range of 1 to 10phr.

A crosslinking accelerator can be used in combination with themercaptotriazine crosslinking agent, which includes organic bases havinga dissociation constant PKa (as defined in Mujio Kotake,Dai-Yuki-Kagaku, separate volume 2 [Manual of Constants in OrganicChemistry], p585–613, published by Asakura Shoten) of at least 7. Asspecific examples of the organic bases, there can be mentionedmonofunctional amines which include strong bases such as laurylamine,guanidine, diphenylguanidine, diorthotolylguanidine, piperidine andpyrrolidine, and ultra-strong bases such as1,8-diaza-bicyclo(5,4,0)undcene-7 (hereinafter abbreviated to “DBU”).The crosslinking accelerator further includes primary, secondary andtertiary aliphatic amines, dibenzylamine, benzylamine andN-methylmorpholine. Of these, diphenylguanidine having a low volatilityand bases having PKa of at least 10 are preferable. Compounds capable ofproducing the organic bases having a PKa of at least 7 can also be usedas the crosslinking accelerator, which include, for example, basic saltssuch as carbonic acid salt, phenol salts, hydrochloric acid salts,sulfuric acid salts and oxalic acid salts of the organic bases; andsodium salt, zinc salt, copper salt, lead salt and piperidine salt ofdithiocarbamic acid.

A crosslinking retardant can also be used, such asN-cyclohexylthiophthalimde.

The amounts of the crosslinking accelerator and the crosslinkingretardant are appropriately determined depending upon the particularkinds thereof, and preferably in the range of 0.1 to 10 phr, morepreferably 0.3 to 5 phr.

A crosslinking aid can also be used in combination with the organic basecrosslinking accelerator. The crosslinking aid includes oxides of metalsof group IIA and group IIB of the periodic table, such as calcium oxide,magnesium oxide and zinc oxide.

(iii) 2,3-dimercaptoquinoxaline Crosslinking Agent

The 2,3-dimercaptoquinoxaline crosslinking agent used in the presentinvention is a compound represented by the following general formula:

wherein each of R¹ through R⁴ independently represents a hydrogen atomor C₁–C₄ alkyl group.

As specific examples of the 2,3-dimercaptoquinoxaline crosslinkingagent, there: can be mentioned quinoxaline-2,3-dithiocarbonate,6-methylquinoxaline-2,3-dithiocarbonate,6-isopropylquinoxaline-2,3-dithiocarbonate and5,8-dimethylquinoxaline-2,3-dithiocarbonate.

The amount of the 2,3-dimercaptoquinoxaline crosslinking agent ispreferably in the range of 0.1 to 10 phr, more preferably 0.5 to 3 phr.

Where the 2,3-dimercaptoguinoxaline crosslinking agent is used, acrosslinking accelerator and a accelerator activator can also be used.The kind and amount of the crosslinking accelerator and a acceleratoractivator may be the same as those described above as for themercaptotriazine crossslinking agent.

(iv) Proportion of Crosslinking Agents

The proportion of the crosslinking agents used may be appropriatelychosen depending upon the particular combination of crosslinking agents.For example, (1) where the sulfur-containing crosslinking agent and themercaptotriazine crosslinking agent are used in combination, the amountof the sulfur-containing crosslinking agent is preferably in the rangeof 0.1 to 5 parts by weight, more preferably 0.3 to 3 parts by weight,per 1 part by weight of the mercaptotriazine crosslinking agent; (2)where the sulfur-containing crosslinking agent and the2,3-dimercaptoquinoxaline crosslinking agent are used in combination,the amount of the sulfur-containing crosslinking agent is preferably inthe range of 0, 1 to 5 parts by weight, more preferably 0.3 to 3 partsby weight, per 1 part by weight of the 2,3-dimercaptoquinoxalinecrosslinking agent, (3) where the mercaptotriazine crosslinking agentand the 2,3-dimercaptoquinoxaline crosslinking agent are used incombination, the amount of the mercaptotriazine crosslinking agent ispreferably in the range of 0.2 to 5 parts by weight, preferably 0,5 to 3parts by weight, per 1 part by weight of the 2,3-dimercaptoquinoxalinecrosslinking agent; and (4) where the three kinds of crosslinking agentsare used in combination, the amount of the mercaptotriazine crosslinkingagent is preferably in the range of 0.2 to 5 parts by weight, preferably0,5 to 3 parts by weight, per 1 part by weight of the2,3-dimercaptoquinoxaline crosslinking agent, and the amount of thesulfur-containing crosslinking agent is preferably in the range of 0.1to 5 parts by weight, more preferably 0.3 to 3 parts by weight, per 1part by weight of the 2,3-dimercaptoquinoxaline crosslinking agent.

Other Ingredients

If desired, the crosslinkable rubber composition of the presentinvention may contain optional ingredients other than theabove-described rubber ingredient, crosslinking agents, crosslinkingaccelerators and others, provided that the object and effect of thepresent invention are not banefully influenced. Such optionalingredients include, for example, reinforcing agents such as varioustypes of carbon black, silica and clay; fillers such as calciumcarbonate; plasticizers; processing aids; and antioxidants. The optionalingredients are not particularly limited and can be selected from thosewhich have heretofore been used for NBR and an epihalohydrin rubber.

Mixing

The crosslinking agent of the present invention can be prepared bymixing or kneading together the above-specified rubber and crosslinkingagents and other optional ingredients by using a mixing means such as aroll, a Banbury mixer and an internal mixer.

The manner and order in which the respective ingredients are mixed arenot particularly limited. For example, (1) a nitrile rubber and ahalohydrin rubber are blended, and then, crosslinking agents and otheringredients are mixedor kneaded together with the rubber blend; and (2)a crosslinking agent and other ingredients are mixed or kneaded togetherwith each of a nitrile rubber and a halohydrin rubber, and then the tworubber compositions are mixed together.

Preferably, the rubber ingredient is first mixed with ingredients whichare relatively difficult to react or thermally decompose under hightemperature conditions, such as a reinforcing agent and a filler, andthen, the resulting mixture is mixed with ingredients, which arerelatively easy to react or thermally decompose under high temperatureconditions, at a low temperature and within a short period of time.

[Crosslinked Rubber Product]

A rubber article comprised of a crosslinked rubber having excellentstatic and dynamic ozone resistance and oil resistance can be made fromthe crosslinkable rubber composition of the present invention.

(i) Molding Method and Crosslinking Method

The molding method and the cross inking method for making thecrosslinked product from the crosslinkable rubber composition of thepresent invention are not particularly limited. The molding method andthe crosslinking method can be conducted concurrently, or successivelyin this order, according to the molding method, the crosslinking methodand the shape of the crosslinked product.

The lower limit of the crosslinking temperature for crosslinking thecrosslinkable rubber composition is preferably 130° C., more preferably140° C. The upper limit thereof is preferably 200° C. If thecrosslinking temperature is too lows the crosslinking time is liable tobe long and the crosslinking density tends to be lowered. In contrast,if the crosslinking temperature is too high, the crossslinking proceedswithin a short time and defective moldings are liable to be produced.

The crosslinking time varies depending upon the particular crosslinkingmethod, crosslinking temperature and shape of the crosslinked product,but is preferably in the range of 1 minute to 5 hours in view of thecrosslinking density and the production efficiency.

The heating method may be appropriately chosen from those which areconventionally employed for crosslinking rubbers and which include pressheating, steam heating, oven heating and hot air heating.

Use

The crosslinkable rubber composition of the present Invention isespecially suitable as material for hoses such as a fuel hose, alubricating oil hose and an air hose, and rubber rolls such as an officemachine roll, a printing roll and paper-making roll.

(i) Hose

For the use of hoses, the crosslinkable rubber composition of thepresent invention is used as material for making a single layer hose, orfor making an inner layer and/or an outer layer of a multi-layer hose.

Hoses are usually made by extruding rubber material through an extruderinto a tubular form. However, a short hose can be made by using a mold,e.g., by injection molding or press molding. In a process of making asingle layer hose, a single layer of the crosslinkable rubbercomposition is formed and, according to the need, a reinforcing layercomprised of a woven or knitted fabric is formed on the surface of thesingle rubber layer. In a process of making a multi-layer hose, forexample, a single inner layer of the crosslinkable rubber composition isformed and, according to the need, a reinforcing layer is formed on thesurface of the single inner rubber layer, and an outer rubber layer ofthe crosslinkable rubber composition is formed further on thereinforcing layer by using an extruder, and if desired anotherreinforcing layer is further formed on the outer rubber layer.

The thus-formed uncrosslinked rubber hose is subjected to crosslinkingto give a hose as a crosslinked product by a method wherein it is heatedwith steam in a vulcanizer, or a method wherein it is cut into apredetermined length, a mandrel is inserted into the cut rubber hose,and the rubber hose is heated with steam in a vulcanizer.

(ii) Roll

For the use of a roll, the crosslinkable rubber composition of thepresent invention is used for the formation of a rubber layer of a roll.

When the roll has a small size, there can be adopted a method whereinthe crosslinkable rubber composition is injected into a mold having ametal core rod placed therein, and then press curing is conducted, and amethod wherein the crosslinkable rubber composition is extruded throughan extruder into a tubular form, a core rod is inserted within thetubular extrudate, and the tubular extrudate is placed in a mold toconduct press curing.

When the roll has a large size, there can be adopted a method whereinthe crosslinkable rubber composition in the form of sheet is woundaround a metal core rod to a predetermined thickness, and then heatedwith steam in a vulcanizer to give a crosslinked rubber roll. When thesheet wound around the metal core rod is relatively soft, the woundsheet can be heated with steam in a vulcanizer, as it is, to give acrosslinked rubber roll. But, when the sheet wound around the metal corerod is relatively rigid, the wound sheet is rolled with, for example, anylon wrapper, and then heated with steam in a vulcanizer to give acrosslinked rubber roll.

The crosslinked rubber roll is surface-polished or surface-treated to bethereby made into a finished article having desired appearance, shapeand precision.

WORKING EXAMPLES

The invention will now be described in detail by the following workingexamples.

EXAMPLES 1–8 AND COMPARATIVE EXAMPLES 1–5

The following rubber ingredients were used.

(i) NBR (1): bound acrylonitrile content: 33.5% by weight, boundbutadiene content: 66.5% by weight, Mooney viscosity (ML₁₊₄, 100° C.):78, tradename “Nipol™ 1042” available from Zeon Corporation

(ii) NBR (2): bound acrylonitrile content: 41.0% by weight, boundbutadiene content: 59.0% by weight, Mooney viscosity (ML₁₊₄, 100° C.):83, tradename “Nipol™ 1041” available from Zeon Corporation

(iii) Epichlorohydrin rubber: epichlorohydrin-allyl glycidyl ethercopolymer, bound epichlorohydrin content: 94% by weight, bound allylglycidyl ether content: 6% by weight, Mooney viscosity (ML₁₊₄, 100° C.):60, tradename “Gechron™ 1100” available from Zeon Corporation

(iv) Polyblend: mixture of 70 parts by weight of NER (boundacrylonitrile content: 33.5% by weight, tradename “Nipol™ 1203”available from Zeon Corporation) and 30 parts by weight of polyvinylchloride.

According to the recipe shown in Tables 1, rubber ingredients were mixedtogether with ingredients other than the crosslinking agents,crosslinking accelerator and accelerator activator by using a 0.8 literBanbury mixer at a preset temperature of 100° C., and then, theresulting mixture was mixed together with the crosslinking agents,crosslinking accelerator and accelerator activator at a presettemperature of 50° C. to prepare a crosslinkable composition.

The ingredients used were as follows.

(i) FEF carbon black: average particle diameter: 51 μm, specific surfacearea: 58 m²/g, pH value: 7.7, tradename “Asahi #60” available from AsahiCarbon K.K.

(ii) SRF carbon black: average particle diameter: 58–94 μm, specificsurface area: 25–30 m²/g, pH value: 7.5–9.0, tradename “Seast™ F”available from Tokai Carbon K.K.

(iii) Clay: specific gravity; 2.62, hard clay, 99.8% passing throughsieve with 325 mesh size, tradename “Dixie Clay™” available from R. T.Vanderbilt Co.

(iv) Calcium carbonate: fatty acid-treated product, average particlediameter: 0.04 μm, specific surface area: 30–32 m²/g, specific gravity:2.55–2.57, pH value: 8.7–9, CaO content: 54.1% by weight, tradename“Hakuennka CC” available from Shiraishi Kogyo K.K.

(v) Magnesium oxide: MgO content: 97.5 by weight, apparent specificgravity 0.25–0.35 g/ml, specific surface area (BET): 60–100 m²/g,tradename “Kyowamag™ 100” available from Kyowa Kagaku Kogyo K.K.

(vi) Lead oxide: #1 lead oxide made by French method, tradename “zincflower #1” available from Sakai Chem. Ind. Co.

(vii) Sulfur: passing through sieve with mesh size 325, tradename“Kinka-mark fine sulfur” available from Tsurumi Kagaku Kogyo K.K.

(viii) 2-ethylhexyl phthalate: Daihachi Kagaku K.K.

(ix) Tetramethylthiuram disulfide: tradename “Nocceler™ TT” availablefrom Ohuchi Shinko kagaku Kogyo K.K.

(x) N-cyclohexyl-2-benzothiazylsulfenamide, tradename “Nocceler™ CZ”available from Ohuchi Shinko kagaku Kogyo K.K.

(xi) 2,4,6-trimercapto-s-triazine; tradename “Zisnet-F™” Sankyo KasetK.K.

(xiii) 6-methylquinoxaline-2,3-dithiocarbamate: tradename “DAISONET™XL-21” available from Daiso K.K.

(xiii) DBU salt: DBU salt of phenol-novolak resin, DBU contents 30% byweight, tradename “U-CAT™ SA 841” available from Sun Avot Co.

Each crosslinkable rubber composition was press-cured at 160° C. for 30minutes (in working examples other than Comparative Example 5) or 15minutes (in Comparative Example 53) to prepare a crosslinked rubbersheet having a thickness of 2 mm. Mechanical properties (tensile test)and static and dynamic ozone resistance of the crosslinked rubber sheetwere evaluated according to JIS K6301.

Static ozone resistance was determined by a method wherein a specimendrawn by 20% was exposed to an atmosphere having an ozone concentrationof 50 pphm at a temperature of 40° C. State of crack occurrence wasobserved after a stated period of time elapsed. Dynamic ozone resistancewas determined by the same method as the above-mentioned method forevaluation of static ozone resistance, except that the crack occurrencestate was observed while a specimen was repeatedly drawn at anelongation of 0 to 30%. In Table 1 and Table 2, “NC” means that crackdid not occur, and other denotations such as “A2” and “B3” means statesof crack occurrence as expressed according to JIS K6301. In Table 2,“broken” means that cracks occurred and further the specimen was broken.Denotation “—” means that a specimen was broken at the previousobservation and thus was not observed. Denotation “NM” means that aspecimen exhibited a too large permanent set and thus the crackoccurrence state could not be evaluated under the specified conditions.

In the tensile test, denotation “TB” means tensile strength at break,“EB” means elongation at break, “M₁₀₀” means modulus at 100% elongation,and “HS” means hardness as measured according to JIS method A.

Permanent set was measured according JIS K6301. Rubber having a toolarge permanent set has no practical use. In general, it is said thatrubber having a permanent set not larger than 10% has a practical use. Aspecimen having a too large permanent set is not suitable for evaluationof its ozone resistance wherein measurement is conducted on a specimenhaving a predetermined elongation given thereto.

Evaluation results of the above-mentioned properties are shown in Table1 and Table 2.

TABLE 1 Examples 1 2 3 4 5 6 7 8 Ingredients (weight parts) Rubberingredient NBR (1) 60 70 60 60 — — — 60 NBR (2) — — — — 60 70 60 —Epichlorohydrin rubber 40 30 40 40 40 30 40 40 Crosslinking agent Sulfur0.5 0.5 0.5 — 0.3 0.3 0.3 0.3 2,4,6-Trimercapto- 1 1. — 1 3 3 — 0.6s-triazine 6-Methylquinoxaline-2,3- — — 1.5 1.5 — — 1.5 0.9dithiocarbamate Crosslinking accelerator Ttetramethylthiuram 1 1 1 — 1 11 0.6 disulfide N-cyclohexyl-2-benzo- 1 1 1 — 1 1 1 0.6thiazylsulfenamide DBU salt — — — — — — 1 — Accelerator activatorMagnesium oxide 3 3 3 3 3 3 3 3 Stearic acid 1 1 1 1 1 1 1 12-ethylhexyl phthalate — — — — 40 40 40 — Other ingredients Calciumcarbonate 5 5 5 5 5 5 5 5 FEF carbon black 50 50 50 50 80 80 80 40 Clay— — — — 30 30 30 — Dry physical properties TB (MPa) 20.4 23.0 21.7 21.113.9 15.5 13.2 21.3 EB (%) 350 370 420 430 280 310 330 320 M_(10C) (MPa)5.68 5.39 4.7 3.63 6.47 5.29 4.7 5.83 HS 73 72 70 68 72 70 69 74Permanent set (%) 2.4 2.9 6.3 6.8 5.8 8.8 10.3 2.2 Static ozoneresistance 24 hours NC NC NC NC NC NC NC NC 72 hours NC NC NC NC NC NCNC NC 168 hours NC NC NC NC NC NC NC NC Dynamic ozone resistance 24hours NC NC NC NC NC NC NC NC 72 hours NC NC NC NC NC NC NC NC 168 hoursNC NC NC NC NC NC NC NC

TABLE 2 Comparative Examples Ingredients (weight parts) 1 2 3 4 5 Rubberingredient NBR (1) 60 60 60 — — NBR (2) — — — 60 — Epichlorohydrinrubber 40 40 40 40 — Polyblend — — — — 100 Crosslinking agent Sulfur 0.5— — 0.3 0.5 2,4,6-Trimercapto-s-triazine — 1 — — —6-Methylquinoxaline-2,3- — — 1.5 — — dithiocarbamate Crosslinkingaccelerator Ttetramethylthiuram 1 — — 1 1.5 disulfideN-cyclohexyl-2-benzo- 1 — — 1 1.5 thiazylsulfenamide Acceleratoractivator Magnesium oxide 3 3 3 3 — Zinc oxide — — — — 5 Stearic acid 11 1 1 1 2-Ethylhexyl phthalate — — — 40 20 Other ingredients Calciumcarbonate 5 5 5 5 — FEF carbon black 50 50 50 50 — SRF carbon black — —— — 60 Clay — — — 30 — Dry physical properties TB (MPa) 19.8 13.8 12.914.5 14.5 EB (%) 480 810 830 410 470 M₁₀₀ (MPa) 2.94 2.45 2.54 5.68 4.7HS 66 68 67 71 70 Permanent set (%) 7.2 25.5 26.3 8.6 8.3 Static ozone24 hours C3 NM NM B3 NC resistance 72 hours Broken NM NM C3 NC 168 hours— NM NM Broken NC Dynamic ozone 24 hours B3 NM NM B4 NC resistance 72hours B4 NM NM Broken A2 168 hours Broken NM NM — Broken

In Comparative Examples 1 to 4, crosslinkable rubber compositions eachhaving one kind of crosslinking agent were prepared, their crosslinkedproducts were made and their properties were evaluated. In ComparativeExample 5, a crosslinkable rubber composition having a conventionalpolyblend was prepared, its crosslinked product was made and itsproperties were evaluated. Crosslinked products made in ComparativeExamples 1 to 4 have a too large permanent set or poor static anddynamic ozone resistances, as compared with the crosslinked producthaving a conventional polyblend made in Comparative Example 5.

In Examples 1 to 8, crosslinkable rubber compositions of the presentinvention were prepared, their crosslinked products were made and theirproperties were evaluated. As seen from the evaluation results, thecrosslinked products exhibit approximately the same permanent set andstatic ozone resistance and improved dynamic ozone resistance, ascompared with the crosslinked product having a conventional polyblendmade in Comparative Example 5.

INDUSTRIAL APPLICABILITY

The crosslinkable rubber composition of the present invention gives acrosslinked product exhibiting approximately the same permanent set andstatic ozone resistance and much improved dynamic ozone resistance, ascompared with a crosslinked product having a conventional NBR/PVCpolyblend. Therefore, the crosslinkable rubber composition is suitableas material for making rolls and hoses.

1. A crosslinkable rubber composition comprising, as rubber ingredient,a nitrile rubber and an epihalohydrin rubber and, as crosslinking agent,at least two kinds of crosslinking agents selected from asulfur-containing crosslinking agent, a mercaptotriazine crosslinkingagent and a 2,3-dimercaptoquinoxaline crosslinking agent.
 2. Thecrosslinkable rubber composition according to claim 1, wherein thesulfur-containing crosslinking agent and the mercaptotriazinecrosslinking agent are contained in combination in amounts of 0.1 to 3phr as the amount of sulfur, and 0.1 to 10 phr, respectively.
 3. Thecrosslinkable rubber composition according to claim 2, wherein theamount of the sulfur-containing crosslinking agent is in the range of0.1 to 5 parts by weight per 1 part by weight of the mercaptotriazinecrosslinking agent.
 4. The crosslinkable rubber composition according toclaim 1, wherein the sulfur-containing crosslinking agent and the2,3-dimercaptoquinoxaline crosslinking agent are contained incombination in an amount of 0.1 to 3 phr as the amount of sulfur, and anamount of 0.1 to 10 phr, respectively.
 5. The crosslinkable rubbercomposition according to claim 4, wherein the amount of thesulfur-containing crosslinking agent is in the range of 0.1 to 5 partsby weight per 1 part by weight of the 2,3-dimercaptoquinoxalinecrosslinking agent.
 6. The crosslinkable rubber composition according toclaim 1, wherein the 2,3-dimercaptoquinoxaline crosslinking agent andthe mercaptotriazine crosslinking agent are contained in combination inamounts of 0.1 to 10 phr and 0.1 to 10 phr, respectively.
 7. Thecrosslinkable rubber composition according to claim 6, wherein theamount of the meroaptotriazine crosslinking agent is in the range of 0.2to 5 parts by weight per 1 part weight of the 2,3-dimercaptoquinoxalinecrosslinking agent.
 8. The crosslinkable rubber composition according toclaim 1, wherein the amounts of the nitrile rubber and the epihalohydrinrubber are 20 to 80% by weight and 80 to 20% by weight, respectively,based on the total weight of the rubber ingredient.
 9. A crosslinkedproduct obtained by crosslinking a crosslinkable rubber composition asclaimed in claim
 1. 10. The crosslinked product according to claim 9,which is a hose.
 11. The crosslinked product according to claim 9, whichis a roll.